182 W. N. THAYER 
It is occupied successively by the headwaters of the Columbia, 
Fraser, Peace, and Liard rivers, nearly all of which leave the trough 
by transverse gorges cut in the adjacent mountains. 
The orographic unit lying between this trench and the western 
edge of the Great Plains includes the Lewis, Livingstone, Mission, 
and a few smaller ranges in the United States, and Daly’s ‘Rocky 
Mountain System” of British Columbia and Alberta.t The topog- 
raphy of these ranges is generally bold, and elevations of more than 
10,000 feet are attained by some of the peaks. These are the true 
front ranges of this part of the Rocky Mountain System.? 
The Purcell trench is also a structural trough. It extends from 
Bonners Ferry, Idaho, northward in line with the courses of the 
Kootenay and Beaver rivers to a point about 200 miles north of the 
International Boundary, where it joins the Rocky Mountain trench. 
The orographic unit that lies between these two trenches com- © 
prises the Coeur d’Alene, Cabinet, Flathead, and Purcell ranges. 
The relief is less than that of the Front Ranges. Few peaks sur- 
pass 7,500 feet, except in a part of the Cabinet Range, and on the 
whole only a small proportion of the summits attain 7,000 feet. 
The more important stream courses trend about northwest and 
form boundaries of the chief subdivisions, each of which bears 
evidence of being a dissected plateau. 
The Selkirk Valley, although not a structural depression, is a 
valley of the first rank. It is drained southward by the Columbia 
River, and extends from a point about 60 miles south of the Inter- 
national Boundary, where the river turns westward to enter the lava 
fields of the Columbia Plateau, to a point about 250 miles north of 
the boundary, where it also joins the Rocky Mountain trench. 
The mountainous unit lying between the Selkirk Valley and the 
Purcell trench embraces Daly’s Selkirk System, which in its exten- 
sion south of the boundary includes the Pend d’Oreille Mountains. 
The topographic character of this unit is similar to that of the unit 
next to the east. In the southern part the mountains are generally 
rounded, and but few of the summits rise above 5,000 feet. North 
1 F. C. Calkins, U.S. Geol. Survey, Bull. 384, p. 12. 
2 Bailey Willis, Bull. Geol. Soc. Am., XIII, pp. 305-52. 
3 F. C. Calkins, loc. cit. 
